1. Field of the Invention
The present invention relates to ceramic multilayer parts and a conductor paste to be used for forming outer conductors for the parts, in particular, to integrated, non reciprocal devices to be mounted on radiocommunication appliances and the like that are used in micro-wave and milli-wave zones, and to a conductor paste to be used for forming outer conductors for the devices.
2. Description of Related Art
With the recent great development of radiocommunication-related technology, there is increasing a demand for electronic parts for use in a high-frequency band of hundreds MHz to several GHz or higher. Radiocommunication appliances such as portable telephones are being small-sized, and high-frequency electronic parts to be mounted on those appliances are required to be small-sized and low-priced. Given that situation, ceramic multilayer parts are produced according to various integration techniques.
To produce ceramic multilayer parts, a ceramic material and a conductor material are co-fired, and the parts thus produced shall have one or more functions. For producing those ceramic multilayer parts, employed is a method comprising laminating a ceramic material and a conductor material into a laminate through printing or sheeting, followed by cutting the resulting laminate into parts having a desired shape and a desired size and then firing them, or followed by firing the laminate and then cutting it into parts having a desired shape and a desired size, and thereafter optionally forming outer conductor parts around the parts.
Therefore, the ceramic multilayer parts are so constructed that they have an inner conductor between the adjacent ceramic layers. As the inner conductor suitable to high frequency, especially to micro-waves, generally employed is Ag, Cu or the like.
However, in order to obtain ceramic multilayer parts having satisfactory properties according to the method noted above, it is considered necessary to prevent the inner conductor from being fused, and it is said that the parts must be fired at a temperature not higher than the melting point of the inner conductor. For these reasons, therefore, it is difficult to use a conductive material having a low resistivity but having a low melting point, such as Ag or Cu, for the inner conductor for ceramic materials to be fired at high temperatures.
Taking the problems noted above into consideration, we the present applicants have assiduously studied, and, as a result, have proposed a composition comprising silver to be an inner conductor and a predetermined oxide added thereto, in Japanese Patent Application No. Hei-8-197156. The inner conductor to be in the composition can be co-fired along with the above-mentioned ceramic materials.
On the other hand, in conventional ceramic multilayer parts, especially irreversible devices such as isolators or circulators, the core element is formed, for example, by sintering garnet-type ferrite (YIG: Y.sub.3 Fe.sub.5 O.sub.19), then integrated with an inner conductor of Ag or Cu foil or the like, and electrically connected with other parts while the Cu foil is led out of the element. Accordingly, in those conventional ceramic multilayer parts, the inner conductor is integrated with the outer electrode, and the parts do not require any additional outer electrode.
However, when the inner conductor is formed while being co-fired along with ceramics in the manner noted above, the ceramic multilayer parts require additional outer electrodes. In that case, it may be taken into consideration to use a paste for inner electrodes as that for outer electrodes. However, for outer electrodes, when they are formed around irreversible devices and the like, they must not lower the Q value of the devices and must have good solder wettability. For their mechanical properties, the outer electrodes must have good adhesiveness to the devices, especially in severe conditions of heat cycle and the like. However, the outer electrodes made of the paste for inner electrodes do not meet the requirements.